The invention relates to crystals having optical properties suitable for use in optical applications, such as harmonic generators, optical parametric oscillators and acousto-optical devices.
When light enters a linear crystal its electric field generates a polarization in the crystal by displacing positive charges in one direction and negative charges in the opposite direction. The bound charges follow the applied field, accelerating and moving in syncrhronism with it, and thus reradiate a light ray similar in direction and frequency to the incident ray.
In linear optical crystals the displacement of the charges is the same for the two opposing directions of the field, but in nonlinear optical crystals, as a result of the crystal structure, the displacement is greater for a field in one direction than in the opposite direction. As a result of the asymmetric motion, the bound charges in nonlinear crystals generate a reradiated wave which is not identical to the driving wave in that it contains small admixtures of higher harmonics of the incident wave, the second harmonic being of particular interest. The efficiency of conversion of the second harmonic depends on the magnitude of the nonlinear optical susceptibility, which is related to the crystal composition and structure, and on the volume of the crystal which is effective in acting as a coherent generator of the second harmonic wave.
In the simplest case, the volume is limited by the fact that due to normal dispersion of the refractive indices of the material, the second harmonic ray propagates more slowly through the crystal than does the fundamental ray. As a result, at a given point in the crystal the harmonic ray derived from the fundamental in say, the first part of the crystal may be out of phase with that derived from the fundamental in a succeeding part of the crystal, resulting in destructive interference of the generated second harmonic wave, and severe limiting of the effective volume for coherent generation of the second harmonic.
In birefringent nonlinear crystals, however, the problem can be overcome by taking advantage of the fact that in such a crystal, there are different effective light propagation velocities, depending on the polarization of the beam and its propagation direction in the crystal. As an example, in a uniaxial negative birefringent crystal an extraordinary ray (i.e. one which is polarized with its E-vector parallel to the crystal c-axis) travels at a faster speed than does the the ordinary ray (i.e. one which is polarized with its E-vector perpendicular to the crystal c-axis.) The difference in speeds increases as the direction of propagation of the extraordinary ray is shifted away from the c-axis and is at a maximum when the extraordinary ray propagation direction is normal to the c-axis. Making use of the fact that through the anisotropic properties of the nonlinear susceptibilities an ordinary fundamental ray generates an extraordinary second harmonic ray, one can increase the speed of the extraordinary second harmonic ray by increasing its angle of propagation to the c-axis until at some angle, known as the "phase-matching angle." .theta. its speed will equal the speed of the fundamental ray propagating as an ordinary ray. Then the contributions Lo the second harmonic ray from the fundamental radiation in all parts of the fundamental ray will be in phase, and the second harmonic ray output and the conversion efficiency will be optimized.
In U.S. Pat. No. 3,792,287 Roland and Feichtner disclose a compound of the formula Tl.sub.3 AsSe.sub.3 made into large optically useful crystals which are birefringement and display nonlinear optical properties in the infrared. They teach that such a crystal can be used in a harmonic generator, an optical parametric oscillator and an optical frequency upconverter. The performance of such a crystal for frequency conversion requires good optical quality, polishing characteristics and mechanical properties. However, the Tl.sub.3 AsSe.sub.3 crystal is relatively very soft and has been difficult to polish and handle. Furthermore, the crystal has not been useful for converting light in the mid-infrared light region.
Roland et al. disclose a crystal of Tl.sub.3 AsS.sub.4 for use in acousto-optical systems in U.S. Pat. No. 3,915,556. This crystal is not a nonlinear crystal and is not suitable for converting light in the mid-infrared light region.
There is a great need for an efficient infrared light source than can operate in the mid-infrared wavelength region. Because there are no efficient light sources in this range, the most practical way to accomplish this goal is to use nonlinear crystals to downshift (using optical parametric oscillation) the output of lasers operating at 1 or 2 .mu.m, or upshift (using second harmonic and higher harmonic generation) the output of the CO.sub.2 laser at 10 .mu.m. The main obstacle to more widespread use of optical parametric oscillation has been the difficulty in growing large, high quality nonlinear crystals with a combination of high nonlinear coefficients and optical and mechanical parameters compatible with high average power operation. The ideal requirements for nonlinear optical materials are very difficult to meet in a single material. Optical homogeneity, laser damage threshold, stability of the compound upon exposure to a laser beam, and ease of fabrication are main concerns. Improved mechanical properties with respect to cutting and polishing will contribute to increasing the surface damage threshold. Another concern for crystal growers is to scale up the crystal size to achieve high efficiency and high power output without sacrificing the optical quality.